1. Field of the Invention
This invention relates to electrical devices.
2. Introduction to the Invention
Many electrical devices comprise two laminar electrodes and, sandwiched between them, an electrical element which may be a conductor, e.g. a resistive element, as for example in a resistor or a varistor, or a non-conductor, as for example in a capacitor. Particularly useful devices of this type are circuit protection devices which comprise a laminate of two laminar electrodes and, sandwiched between the electrodes, a laminar resistive element which exhibits PTC behavior. The resistive element may be composed of conductive polymer (this term being used to denote a composition comprising a polymer and, dispersed, or otherwise distributed, therein, a particulate conductive filler) or a ceramic, e.g. a doped barium titanate. When a conductive polymer is used, such devices are generally prepared by stamping (or otherwise cutting) a plurality of the devices out of a laminate of a sheet of the conductive polymer between two metal foils. When a ceramic is used, such devices are usually prepared by applying liquid electrode material to the major surfaces of a preformed laminar resistive element, and solidifying the liquid electrode material.
The products of such processes can sometimes be used without the addition of electrical leads, for example by installation between two spring-loaded terminals. In most cases, however, an electrical lead must be secured to each of the laminar electrodes, so that the device can be connected to other components of a circuit, e.g. mounted on a circuit board. The addition of leads is an additional expense and usually involves heating (e.g. during soldering or welding) which can cause damage, particularly to conductive polymer elements. The latter problem is particularly severe when a conductive polymer is heated a second time when the leads are connected to other circuit elements, in particular when the leads are connected to a printed circuit board by a soldering process. A further problem which can arise when such devices are to be mounted on a printed circuit board is that they protrude further from the board than is desirable.
We have now realized, in accordance with the present invention, that when at least one of the laminar electrodes of an electrical device as described above is to be connected to an electrical conductor on an insulating substrate, in particular a printed circuit board, then by appropriate modification of the device and/or of the configuration of the electrical conductor on the substrate, the connection can be made either without the need for a lead (or other connecting member), or with the aid of a connecting member which is electrically connected to the electrode in the same step as it is electrically connected to the conductor on the substrate. We have also realized, in accordance with the present invention, that such modification can also be extremely valuable for electrical devices which are connected via elastically deformed terminals. The invention will be described herein chiefly by reference by circuit protection devices comprising a laminar PTC resistive element sandwiched between, and preferably contacted directed by, two laminar electrodes, but it is also applicable to other devices comprising two laminar electrodes and another type of laminar element sandwiched between them.
Typically, the devices of the invention have, in addition to a main portion having a normal configuration (typically a simple geometric shape such as a rectangle or a circle), at least one connection leg which extends away from the main portion and which comprises an extension of the PTC resistive element and an extension of one of the electrodes secured to the PTC element. The connection leg can, but preferably does not, also contain an extension of the other electrode.
In one use of such modified devices, the device is mounted on a circuit board by inserting the end of the connection leg into an aperture in the board and soldering the electrode to a metal conductor which is secured to the board and which leads to the aperture. If the connection leg does not include extensions of both electrodes, then the solder connection can be carried out in conventional ways. If the leg does include extensions of both electrodes in the region of the aperture, then care must be taken to ensure that the solder connects only the desired electrode to the metal conductor. For this purpose, the metal conductor can lead to one side only of the aperture and preferably can be substantially narrower than the aperture and the face of the connection leg to which it is soldered. A circuit board having a metal conductor and an aperture of this kind is believed to be novel per se and as such forms a part of the present invention. Generally, a device which is to be mounted in this way will be modified so that it has two connection legs of the kind described, extending in the same direction from the main portion of the device, so that the legs can be mounted in adjacent apertures in the same circuit board. The legs preferably have distal portions which are inserted into the apertures in the board, and intermediate stand-off portions which cannot pass through the apertures and which ensure that the main part of the device is spaced apart from the board.
As discussed above (and in greater detail below), when the device is to be mounted in apertures in a circuit board, the connection leg can comprise extensions of both electrodes; preferably, however, the connection leg includes a bridge portion which extends across the full width of the leg and which includes only one of the electrodes, so that the device can be soldered to a conventional circuit board in the conventional way. When the connection leg is of the latter kind, the device can be mounted in the apertures of a circuit board as outlined above, or can be mounted flat on top of a circuit board.
To mount the device flat on top of a circuit board, the device is placed on the substrate with the electrode which extends into the connection leg on top. The bottom electrode is connected directly to a conductor on the board. The top electrode is connected to another conductor on the board by means of a connection member secured to the top electrode and extending downwards below the lower face of the PTC (or other) element. In a particularly preferred embodiment, the connection member is a transverse conductive member which passes through the PTC (or other) element; such a transverse member is often referred to herein as a xe2x80x9ccross-conductorxe2x80x9d. The connection member is preferably electrically connected to the upper electrode before it is electrically connected to the conductor on the board; alternatively, both electrical connections can be made simultaneously. If there are two connection legs, one containing an extension of one of the electrodes only, and the other containing an extension of the other of the electrodes only, the device can be placed on the board with either electrode on top, and if the device is symmetrical, the connections to the device and the subsequent operation of the device can be the same.
The novel devices of the invention can be made by securing electrodes of appropriate shapes to resistive elements of the desired final shape; or by securing electrode precursors of appropriate shapes to resistive elements which are larger than the desired final shape, and then dividing the assembly into a plurality of devices of the desired final shape or shapes; or by preparing a plurality of devices of the desired final shape or shapes by division of a simple laminate of constant cross-section and, if desired or necessary, and before or after the division, removing unwanted portions of one or both of the electrodes. Such removal can be effected for example by milling or by etching. Preferably such removal of unwanted portions of the electrodes removes little or none of the PTC resistive element, which provides desirable physical strength to the connection leg. Preferably also, when, as is preferred, the connection leg includes an extension of only one of the electrodes, the leg does also include a residual portion of the second electrode. The residual portion is not electrically connected to the main portion, but provides valuable physical properties, including strength and resistance to deformation when connection to the first electrode is made by a spring clip or other elastically deformed terminal. A preferred process for preparing devices of the invention is described in copending, commonly assigned U.S. patent application Ser. No. 08/257,586, now abandoned in favor of U.S. application Ser. No. 08/808,135, now U.S. Pat. No. 5,864,281, incorporated by reference herein.
The devices of the invention which contain at least one cross conductor which passes through the resistive element can be made by processes in which the various operative steps are carried out on an assembly which corresponds to a plurality of devices in both the longitudinal and the lateral dimension, and which, as the final step of the process, is divided into a plurality of devices. The ability to prepare devices in this way becomes increasingly important as the size (and, therefore, resistance) of the device decreases and this invention is especially valuable for preparing devices which are to be mounted on circuit boards and in other situations in which the smaller the size and resistance of the device, the better. For example, such a process can be used to make circuit protection devices having a surface area of about 0.02 inch2 (13 mm2) or even less.
The various steps of the process are preferably carried out at a temperature substantially below the melting point of the PTC element, in order to minimize changes in its electrical properties.
In one preferred aspect, this invention provides a novel assembly which comprises
(1) a PTC resistive element which
(a) is composed of a resistive material which exhibits PTC behavior,
(b) has a first face and a second face, and
(c) defines an aperture which runs between the first and second faces;
(2) a transverse conductive member which
(a) lies within the aperture defined by the PTC element,
(b) runs between the first and second faces of the PTC element, and
(c) is secured to the PTC element; and
(3) a first laminar conductive member which (a) is secured to the first face of the PTC element and (b) is physically and electrically connected to the transverse conductive member.
This novel assembly can be
(i) an electrical device which is ready for connection (the first laminar conductive member then providing the first electrode, and the device including also a second electrode which is not electrically connected to the cross-conductor), or
(ii) a structure which (if necessary after further processing) can be divided into a plurality of electrical devices, each of the devices containing at least one cross-conductor.
In another preferred aspect, this invention provides an electrical assembly which comprises
(A) a printed circuit board including first and second conductive traces on a surface thereof, and
(B) an electrical device which comprises
(1) a laminar PTC resistive element which
(a) is composed of a resistive material which exhibits PTC behavior, and
(b) has a first face and a second face;
(2) a first laminar electrode which is secured to the first face of the PTC element;
(3) a second laminar electrode which is secured to the second face of the PTC element;
(4) an additional laminar conductive member which (a) is secured to the second face of the PTC element and (b) is spaced apart from the second electrode;
the PTC element, the first electrode and the additional laminar conductive member defining an aperture which runs between the first electrode and the additional conductive member, through the PTC element; and
(5) a transverse conductive member which
(a) lies within the aperture, and
(b) is physically and electrically connected to the first electrode and the additional conductive member;
said electrical device being placed on the printed circuit board and parallel thereto, with the first conductive trace physically and electrically connected to the additional conductive member, and the second conductive trace physically and electrically connected to the second electrode.
In another preferred aspect, this invention provides a method of making electrical devices which comprises
(A) providing an assembly which corresponds to a plurality of the electrical devices and which comprises
(1) a laminar PTC resistive element which (i) is composed of a resistive material exhibiting PTC behavior, and (ii) has a first face and a second face.
(2) a first laminar conductive member which is secured to the first face of the PTC element, and
(3) a second laminar conductive member which is secured to the second face of the PTC element;
(B) making a plurality of apertures through the thickness of the assembly provided in step (A), the apertures being arranged in a regular pattern;
(C) simultaneously with step (B), or after step (B), placing a plurality of transverse conductive members within the apertures, in electrical contact with the first laminar conductive member;
(D) removing predetermined portions of at least one of the first and second conductive members; and
(E) after steps (A) to (D), dividing the assembly into a plurality of electrical devices, each device comprising
(1) a part of the PTC resistive element,
(2) a part of the first laminar conductive member, said part providing a first electrode,
(3) a part of the second laminar conductive member, said part providing a second electrode,
(4) a residual part of the second laminar conductive member, and
(5) at least one transverse conductive member which electrically connects the residual part and the first electrode.
In this method, step (D) can be carried out before or after step (B) or before or after step (C).
When the PTC material is a conductive polymer, this method preferably makes use of a novel assembly which forms part of the present invention, namely an assembly which comprises
(1) a laminar PTC resistive element which (i) is composed of a conductive polymer exhibiting PTC behavior, and (ii) has a first face and a second face,
(2) a first laminar conductive member which is secured to the first face of the PTC element, and
(3) a second laminar conductive member which is secured to the second face of the PTC element;
the PTC element and the first and second laminar conductive members defining a plurality of apertures which pass through the thickness of the assembly, and the apertures being arranged in a regular pattern.
In this assembly, preferably
(a) the apertures are arranged in a plurality of straight lines, and
(b) the first laminar conductive member is in the form of a plurality of strips which are parallel to each other and to the lines of apertures.
It is particularly preferred that
(a) each strip of the first conductive member contains two adjacent lines of apertures, and
(b) the second laminar conductive member is also in the form of a plurality of strips which are parallel to each other and to the lines of apertures, with each strip containing two adjacent lines of apertures, one of said lines of apertures falling within a first strip of the first conductive member and the other line of apertures falling within a second strip of the first conductive member.